The rendering of animal tissues such as those remaining after removing the edible meat in a meat processing plant, is generally designed to produce three major fractions, tallow, protein/bone meal (more generally termed meat and bone meal) and a waste stream of water. Both tallow and the meat and bone meal are commercially significant products. Sterilization is an important influence on quality and hence on the price that can be obtained.
Rendering of animal tissues by heat is carried out principally by two methods: dry or high temperature rendering; and wet or low temperature rendering.
In the high temperature process the raw material is heated, separated into the tallow, the fat-free solid and water phases and dried in the same operation. This process suffers from drawbacks: 10-15% residual fat in the cooked product; the resultant tallow must be thoroughly washed to ensure the product is of a premium grade; the high temperature can result in a degradation of the nutritive value of the tallow; and the process requires a high energy input.
In the low temperature process the raw material is heated to 50 to 60° C. to form a hot slurry, and the solids are separated from the liquids. The temperature must be sufficient to liquefy the tallow without evaporating the water. The solids stream contains protein and water and the liquids stream contains tallow, soluble protein and water. The liquid stream is further separated into a heavy phase and a light phase, with the light phase having an increased amount of tallow. The heavy phase and the solid phase are combined to form a meal and bone product. The disadvantages of this process are: the water (stickwater) has a significant tallow content of between 10-15%; and the adequacy of degree of sterilization of the tallow has been questioned.